The present invention relates to improving the input return loss in RF amplifiers. More specifically, the impedance of an amplifier module including a 3 dB coupler at the input is maintained substantially constant while removing one of the two amplifiers normally connected to the output terminals of a 3 dB coupler. A preferred embodiment replaces the removed amplifier with an electrical circuit with an impedance that is substantially equivalent to the input impedance of the non-removed amplifier. Another preferred embodiment uses the amplifier module including a 3 dB coupler with an electrical circuit in place of one of the amplifiers in a cascaded-stage power amplifier circuit for a television transmitter.
RF amplifier input network designs must typically satisfy a number of constraints: (a) maintain required operational performance, such linearity, gain flatness, and maintaining sufficient signal with a specified slope at the active device input terminal, (b) maintain operational stability over the design operating range while compensating for incidental effects, such as am to am and am to pm distortion, (c) satisfy overall physical constraints for the amplifier, such as size, shape, weight, and cost considerations, and (d) provide a match to some nominal system interface impedance, which is typically 50 ohms for an RF system. These design criteria are typically used in television transmitter amplifier networks.
The requirement to match the nominal system interface impedance with sufficient accuracy is typically the hardest to achieve. Matching the interface impedance becomes an even more daunting task for amplifier networks comprising cascaded stages. For such systems, it is very difficult to match the interface impedance of each stage while efficiently achieving, in terms of amplifier efficiency, size, shape, weight, and cost, the necessary amplification at each stage. Overall performance of the cascaded amplifier networks, when compared with the results expected from the sum of individual stage performances, degrades rapidly when interstage impedance is not maintained.
One prior art solution to the problem of maintaining interstage impedance is the use of a quadrature hybrid combined amplifier for each stage of a cascaded-stage amplifier system. Quadrature hybrid combined amplifiers are known in the art and are described in detail in Anaren""s 1997 Product Catalog, pp. 60-73, Anaren Microwave, Inc., which is hereby incorporated herein by reference. These quadrature hybrid combined amplifiers are used in cascaded-stage power amplifier networks for television transmitters.
An example of a prior art quadrature hybrid combined amplifier is shown diagrammatically in FIG. 2. The prior art quadrature hybrid combined amplifier 200, also referred to herein as an xe2x80x9camplifier palletxe2x80x9d or xe2x80x9cpalletxe2x80x9d, comprises the 3 dB coupler 210 at the input of the device acting as a divider, an amplifier for each of the output terminals of the 3 dB coupler 210, and the 3 dB coupler 240 at the output of the pallet 200 acting as a combiner.
A typical 3 dB coupler, as is known in the art, may input a signal at one input terminal and produce, as a function of the input signal, an in-phase and a quadrature signal, relative to the input signal, each at a separate output terminal and each at approximately one-half of the power of the input signal. Generally, for example, when used at the input of the pallet 200, the input 3 dB coupler 210 receives the input signal 201 on one input terminal while the other input terminal is terminated by the input terminator 215. The 3 dB coupler 210 produces an in-phase signal and a quadrature signal, which are sent to the amplifier circuits 220 and 230, respectively. The amplifier circuits 220 and 230 produce an amplified version of the in-phase and quadrature signals, respectively, which are combined in the output 3 dB coupler 240. The output 3 dB coupler 240 produces an amplified, recombined input signal on one output terminal while hi the other output terminal is terminated. While this description provides a general idea of the signal flow paths through the pallet 200 in FIG. 2, a more complete description of FIG. 2 will be provided below.
3 dB hybrids have the desirable property of high input return loss at the common driven input port, provided the load impedances at the in-phase and quadrature output terminals are identical. Taking the example of the input 3 dB coupler 210 of FIG. 2, the impedance of the amplifiers 220 and 230 at the in-phase and quadrature output ports, respectively, is typically different than the nominal system impedance. However, as long as the impedance of each of the amplifiers is identical, essentially all of the energy reflected by the amplifiers at the in-phase and quadrature output ports of the input 3 dB coupler is absorbed at the terminated port of the 3 dB coupler 210. This results in nominal system impedance at the non-terminated input port of the 3 dB coupler. Return losses of better than 20 dB are typically achieved over the two to one and greater bandwidths of commercially available 3 dB hybrids.
While placing two amplifiers of identical impedance at the in-phase and quadrature output ports of the input 3 dB coupler of an amplifier stage effectively matches the impedance of the stage with the nominal system impedance, such a solution may be inefficient in terms of the stage""s cost, complexity, size, and overall efficiency if the amplification capacity with two amplifiers is more than is needed. In a cascaded-stage amplifier network, the amplification capacity of two amplifiers are not always needed in every stage and there is not an infinite gradation of available active to semiconductor devices at a corresponding cost gradation to allow a convenient scaling of the two amplifier approach to any required design capacity. In that regard, the elimination of one of the two amplifiers can supply the appropriate scaling to match the required design capacity. Typically, some of the amplifier modules in the initial stages of a cascaded-stage amplifier network, such as a driver stage, do not need the two amplifier capacity. Using an amplifier module with two amplifiers in the driver stage may not be cost effective, may render the driver stage too large physically to fit into a desired space, and may underutilize the amplification capacity available thereby reducing the overall efficiency of the cascaded-stage amplifier network while unnecessarily increasing the complexity of the system.
The present invention solves the above-mentioned drawbacks of the prior art by replacing one of the amplifiers at either the in-phase or quadrature output port of the input 3 dB coupler with an electrical circuit (xe2x80x9cdummy networkxe2x80x9d) that emulates the input impedance of the non-replaced amplifier. A preferred embodiment matches the impedance of the dummy network with the input of the input network of the non-replaced amplifier, thereby maintaining the impedance balance between the in-phase and quadrature output ports of the 3 dB coupler. Another preferred embodiment uses the above-described matching dummy network configuration for television transmitters that may be used to transmit COFDM and/or 8VSB signals that may be in the 470 MHz to 860 MHz frequency range.
The dummy network that replaces one of the amplifiers may be the same as, or similar to, the input network of the non-replaced amplifier and may comprise a simple reactive network with a resistive/reactive termination to simulate the amplifier""s active device load. The dummy network may replace either the amplifier at the in-phase output terminal or the amplifier at the quadrature output terminal. So long as the impedance of the dummy network substantially matches the input impedance of the non-replaced amplifier, the input impedance of the 3 dB coupler, and therefore the impedance of the amplifier stage, will remain at the nominal interstage impedance. Typical interstage impedance values for cascaded-stage amplifier networks in television transmitters is 50 ohms but it is to be understood that the effectiveness of the present invention is not limited to systems with 50 ohm impedance. The present invention is also effective in systems where the nominal interstage impedance is 75 ohms, such as for a CATV (cable television) system, and 300 ohms (balanced) such as for television receiver antenna circuits. It is to be understood that the above examples are not limiting and that the applicability of the present invention is not limited to any particular interstage impedance value.
The prior art amplifier circuit design method required that the circuit designer faced with the task of designing a circuit or an amplifier stage with only a single amplifier, had to design the amplifier with the capacity to amplify the input signal the desired amount as well as design an input network for that amplifier that satisfied all of the multiple criteria mentioned above: flat signal gain with frequency, stability, low am to am and am to pm distortion, and good input impedance relative to the nominal system impedance. By far the most difficult criteria to achieve is matching the input impedance with the nominal system impedance.
The present invention allows for the power amplifier circuit designer to design an amplifier stage with a single amplifier without having to worry that the impedance of the stage will not match the nominal system impedance. The use of a 3 dB coupler with a dummy network attached to either the in-phase or quadrature output terminal where the impedance of the dummy network substantially matches the impedance of the single amplifier greatly relieves the design burden of the amplifier circuit. It is much easier to design an amplifier input network that satisfies all of the design constraints but the constraint of matching nominal system impedance and a dummy network to mimic the input impedance of that amplifier than it is to design a single amplifier with an input network that satisfies all of the design constraints.
While the present invention sacrifices the amplification capacity of the replaced amplifier by replacing the amplifier with a dummy network, the amplification capacity that is lost is typically underutilized. Therefore, the overall performance of the cascaded-stage amplifier network does not suffer. The present invention also sacrifices 3 dB of gain compared to prior art systems with two amplifiers. However, the loss of 3 dB of gain is not a detriment to the operation of the cascaded-stage amplifier network provided that the overall design gain requirements of the network are met.
Accordingly, it is an object of the present invention to obviate many of the above problems in the prior art and to provide a novel system and method for improving the input return loss in RF amplifiers.
It is another object of the present invention to provide a novel system and method for amplifying only one of the two output quadrature components of an input signal to a 3 dB coupler in an amplifier module while substantially maintaining a constant input impedance for the 3 dB coupler.
It is yet another object of the present invention to provide a novel system and method for improving the input return loss of an RF amplifier pallet including a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier.
It is still another object of the present invention to provide a novel system and method of operating a hybrid combined amplifier module comprising a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier.
It is a further object of the present invention to provide a novel system and method for amplifying either a COFDM or an 8VSB signal in the 470 MHz to 860 MHz range in a cascaded-stage power amplifier where each stage includes a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input impedance of the non-replaced amplifier.
It is yet a further object of the present invention to provide a novel system and method for improving the input return loss of an RF amplifier pallet in a cascaded-stage power amplifier circuit for a television transmitter including a 3 dB coupler by replacing one of the two amplifiers connected to the output of the 3 dB coupler with an electrical circuit of substantially equivalent impedance to the input network of the non-replaced amplifier.
It is still a further object of the present invention to provide a novel system and method for maintaining, at a predetermined value, the impedance of an amplifier module comprising a 3 dB hybrid coupler with two output terminals each passing one of the two quadrature output signals to a separate amplifier, by replacing one of the amplifiers with electrical circuit with an impedance that substantially matches the input impedance of the non-replaced amplifier.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.